t2402: ensure locked worktree is properly cleaned up

t2402: ensure locked worktree is properly cleaned up

c57b3367be (worktree: teach `list` to annotate locked worktree,
2020-10-11) introduced a new test to ensure locked worktrees are listed
with "locked" annotation. However, the test does not clean up after
itself as "git worktree prune" is not going to remove the locked worktree
in the first place. This not only leaves the test in an unclean state it
also potentially breaks following tests that rely on the
"git worktree list" output.

Let's fix that by unlocking the worktree before the "prune" command.

Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Rafael Silva <rafaeloliveira.cs@gmail.com>
Reviewed-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>